The present invention generally relates to laser diodes and more particularly to an improvement of a laser diode having a stripe structure.
With the widespread use of laser diodes in various fields of human society, such as for optical storage of information in optical disk systems, optical reading of bar codes in POS systems, optical recording of images in printers, and the like, there is an increasing demand for a laser diode that produces short wavelength optical beam in the visual wavelength band. With the use of such short wavelength radiation for the optical beam, one can increase the storage capacity of the optical information storage system. Further, the use of visible light is advantageous in other applications such as POS systems.
In the optical information storage systems that record and reproduce information on and from a recording media by means of a finely focused optical beam, the laser diode is required to have small astigmatism in addition to the usual requirement of low threshold current, high output power and high reliability. In the optical information storage systems in particular, the optical beam is required to have a very small round beam shape when focused on a recording medium such as an optical disk. When there is a large astigmatism in the laser diode, the optical beam does not form the desired round beam spot but forms an elongated spot wherein the direction of elongation changes depending on the focusing state. In order to eliminate or minimize the problem of astigmatism, various efforts have been made so far.
FIG. 1 shows the basic structure of a typical conventional laser diode of the so-called ridge type that produces an output optical beam in the visible wavelength region.
Referring to FIG. 1, the laser diode is constructed on the (100)-oriented upper major surface of a GaAs substrate 101 doped to the n-type. There, a buffer layer 102 of n-type GaAs is grown epitaxially on the (100)-oriented surface of the GaAs substrate 101, and an intermediate layer 103 of n-type InGaP is grown further on the buffer layer 102 epitaxially.
On the intermediate layer 103, a clad layer 104 of n-type InGaAlP is grown epitaxially, and an active layer 105 of undoped InGaP is grown epitaxially on the clad layer 104. The active layer 105 in turn is covered by a clad layer 106 of p-type InGaAlP, and an etching stopper layer 107 of p-type InGaP is grown on the clad layer 106. Further, a clad layer 109 of p-type InGaAlP and an intermediate layer 110 of InGaP are grown consecutively to form a layered semiconductor body, and a silicon oxide layer (not shown) is deposited further on the layer 110. Next, the silicon oxide layer is patterned in correspondence to the ridge structure to be formed to form a patterned silicon oxide mask, and the layered semiconductor body obtained previously is subjected to a wet etching process while using the patterned silicon oxide mask. Thereby, a ridge structure including the layers 109-110 is formed on the etching stopper layer 107. Further, while using the same silicon oxide mask, a deposition of n-type GaAs is made such that the foregoing ridge structure is laterally supported by a pair of n-type GaAs regions formed as a result of the foregoing deposition of n-type GaAs.
In the laser diode of FIG. 1, the layers 109-111 form a ridge structure supported laterally by the n-type GaAs regions 108a and 108b as already noted, and the ridge structure thus constructed forms a loss guide structure for guiding therethrough the optical beam produced in the active layer 105. There, the GaAs regions at both sides of the ridge structure absorbs the optical beam and changes the refractive index in response thereto. It should be noted that the band gap of GaAs is much smaller than the band gap of InGaP or InGaAlP. Further, such a ridge structure, supported laterally by the p-type GaAs regions 108a and 108b, causes a confinement of the drive current within the ridge. Thereby, the laser diode of FIG. 1 is characterized by a low threshold current. By using InGaP for the material of the active layer 105, one can realize a laser oscillation at the wavelength of about 680 nm. Thus, the laser diode of FIG. 1 is capable of producing output optical beam with a visible wavelength.
The laser diode of FIG. 1, however, has a problem in that there appears an astigmatism in the optical beam produced from the device. More specifically, there arises a discrepancy in the focal point of the optical beam that is emitted at the edge surface of the laser diode with a horizontal plane of beam divergence and the optical beam that is emitted at the same edge surface with a vertical plane of beam divergence. FIG. 2 shows such a situation wherein two focal points, f.sub.1 and f.sub.2, exist in the optical beam produced from the laser diode. There, the optical beam spreads in the vertical plane from the first focal point f.sub.1, while the optical beam spreads in the horizontal plane from the second focal point f.sub.2, wherein the focal point f.sub.2 is offset from the focal point f.sub.1 by several microns. Associated with such an offset in the focal point, the beam spot of the optical beam has an elongated elliptical shape that is not desirable for optical storage of information such as the optical disk systems as already described. It will be noted that the direction of elongation of the elliptical beam spot changes depending on the focusing state of the optical beam.
The foregoing astigmatism occurring in the laser diode of FIG. 1 is caused mainly by the difference in the degree of optical confinement between the vertical plane and the horizontal plane. As long as the loss guide structure that lacks the refractive structure for efficient lateral optical confinement is used, one cannot avoid the problem of astigmatism. In order to overcome or minimize the astigmatism, the inventors of the present invention have proposed previously in the European Laid-open Patent Publication EP 0 454 476 that corresponds to the U.S. patent application Ser. No. 691,620 now abandoned and Ser. No. 892,680 that is a file-wrapper continuation of the former, a laser diode having a stripe structure wherein an active layer is provided to extend as a stripe in correspondence to a mesa structure that is formed on a substrate on which the laser diode is constructed.
FIG. 3 shows the laser diode proposed previously by the present inventors.
Referring to FIG. 3, the laser diode is constructed on a GaAs substrate 201 doped for example to the p-type. The substrate 201 has a (100)-oriented upper major surface, and a mesa structure 201a that is characterized by a (100)-oriented upper major surface and a pair of (111)B-oriented side walls, is formed on the upper major surface as illustrated. It should be noted that the (100) surface of the mesa structure 201a extends in the longitudinal direction of the laser diode and forms the basis of the stripe structure.
On the substrate 201 thus shaped, a current confinement layer 202 of n-type GaAs is grown epitaxially while protecting the upper major surface of the mesa structure 201a by a mask such as silicon oxide. When an epitaxial process is applied to the (111)B surface that extends obliquely to the (100) surface of the GaAs substrate 201, it is known that a (311)B surface develops preferentially because of the slow rate of crystal growth in this crystal orientation. In other words, the epitaxial layer 202 thus grown is characterized by the well-developed (311)B surface that extends obliquely to the (100) surface of GaAs at both sides of the mesa structure 201a. Thereby, the (311)B surface forms another mesa structure that extends coincident to the stripe structure of the laser diode.
After the layer 202 is formed and the mask removed, a buffer layer 203 of p-type GaAs is grown epitaxially for providing an improved crystal surface for the subsequent epitaxial processes, and an intermediate layer 204 of p-type InGaP corresponding to the intermediate layer 110 is grown further on the buffer layer 203. Further, a clad layer 205 of p-type InGaAlP is grown epitaxially on the intermediate layer 204, and an active layer of undoped InGaP is grown epitaxially on the clad layer 205.
On the active layer 205, a clad layer 207 of n-type InGaAlP is grown epitaxially, and an intermediate layer 208 of n-type InGaP corresponding to the intermediate layer 103 of FIG. 1 is grown further thereon. Further, a contact layer 208 of n.sup.+ -type GaAs is grown on the intermediate layer 110 epitaxially, and upper and lower electrode layers (not shown) are provided respectively on the upper major surface of the contact layer 209 and the lower major surface of the substrate 201. The epitaxial layers are grown by the MOCVD process for exact control of composition, and dopants are incorporated into the epitaxial layers as necessary during the epitaxial process. Usually, Zn is used for the p-type dopant, while Se or Si is used for the n-type dopant. Zn may be incorporated by admixing dimethylzinc ((CH.sub.3).sub.2 Zn) to the source gas of the epitaxial layers, while Se is incorporated by admixing hydrogen selenide (H.sub.2 Se). When Si is used, a gas of silane (SiH.sub.4) or disilane Si.sub.2 H.sub.6) is used.
In operation, a forward bias voltage is applied across the upper and lower electrodes to inject carriers into the active layer 206. In the illustrated example, holes are injected into the p-type substrate 201 and transported to the active layer 206 through the mesa structure 201a due to the current confinement that is achieved by the n-type GaAs layer 202 at both sides of the mesa structure 201a. Thereby, the holes are injected preferentially at the central part of the clad layer 205 and transferred further to the active layer 206 for recombination with electrons that are injected from the upper electrode and transported to the active layer 206 via the layers 209-207. Such a recombination of electrons and holes initiates the well known stimulated emission process, and the stimulated emission process causes an amplification of the optical beam when reflectors are provided at both longitudinal ends of the laser diode for reflecting the optical beam back and forth through the laser diode. Thereby, one obtains a laser oscillation as is well known in the art.
In the structure of the laser diode thus fabricated, it should be noted that the layers 203 through 209 are all formed in conformity with the surface morphology of the second mesa structure in that each layer includes an elongated stripe region characterized by a (100) surface and extending in correspondence to the (100) surface of the mesa structure 201a. There, each stripe region is laterally defined by a pair of lateral regions located at both sides of the elongated (100) stripe surface and characterized by the (311)B surface in correspondence to the (311)B surface of the layer 202. Of course, the (100) surface is flat and extends in parallel with the upper major surface of the mesa structure 201a, while the (311)B surface extends obliquely to the (100) surface. Because of the lateral confinement of the optical beam in the stripe region of the active layer wherein the recombination of the carriers occurs predominantly, the problem of astigmatism is successfully eliminated in the device of FIG. 3.
In such a structure wherein each epitaxial layer includes crystallographically non-equivalent surfaces, the nature or property of the epitaxial layers changes depending on the orientation of the crystal surface.
In the laser diode of FIG. 3, it will be noted that the clad layer 205 is formed of three distinct regions characterized by respective, three crystallographically distinct surfaces, the first region characterized by the (100) surface and the second and third regions characterized by the (311)B surfaces, wherein the second and third regions are located at both sides of the first region and extend in the longitudinal direction of the laser diode together with the first region. As will be discussed in detail later, it was discovered that the concentration level of the dopant changes depending on the first through third regions of the clad layer 205. More specifically, the concentration level of Zn is higher in the second and third regions, i.e., characterized by the (311)B surface, as compared with the first region, i.e., characterized by the (100) surface. Associated with such a variation in the concentration of the dopant, there appears a variation in the carrier density such that the density or concentration level of the holes is higher in the second and third regions as compared with the first region. This indicates that the resistivity of the clad layer 205 increases to a higher value in the first region as compared with the second and third regions located at both sides thereof because of the reduced concentration of the carriers. Thus, there is a tendency that the injected current flows preferentially through the oblique region of the clad layer 205 rather than the stripe region having the (100) surface as indicated by arrows in FIG. 3. In other words, drive current avoids the strip region of the active layer 206 where the recombination occurs, and the laser diode of FIG. 3 has a problem of poor current confinement and hence a low efficiency of laser oscillation.
Further, the conventional laser diode of FIG. 3 has a problem of large resistivity due to the relatively small Zn content in the epitaxial layer. More specifically, the small Zn content in the epitaxial layer causes the problem of small concentration of holes in the p-type layers, and the conventional laser diode has suffered from the problem of large resistivity and limited operational power. It should be noted that the laser diode would generate intolerable heat due to the large resistivity of the p-type layers, particularly when the injection current is increased. It is thought that this problem arises from the large vapor pressure of Zn that is in equilibrium with the crystal phase during the MOCVD process. There, Zn tends to concentrate more in the vapor phase than in the crystal as a result of the evaporation, and such a tendency is enhanced when the temperature of the epitaxial process is increased. As it is desirable to use high temperature for obtaining high quality crystal layer in the epitaxial process, the foregoing tendency contradicts with the requirement of large dopant concentration level in the epitaxial layers of the laser diode.